The proliferation of localized data-processing and data-handling equipment has spurred provision of data signal communication via inexpensive communication links between these various equipments linked together to form local area network systems. Serial transmission of data on only one inexpensive signal line such as a twisted pair of wires or a coaxial cable is a cost-effective way of linking data equipment into a local area network.
Data signals sent on signal lines are subject to distortion and electrical interference, in the form of impulse noise and the like. Recovery of data information from a distorted and noisy transmitted data signal by a receiver requires the use of a local clock signal at the receiver. Providing a separate clock signal at the receiver site which is synchronous with the transmitted data is a significant problem. Transmitting a separate clock signal on a separate line is an expensive solution. Conventional non-return-to-zero (NRZ) data signals usually contain a direct-current dc component, which is particularly strong on long strings of ONEs or ZEROs. For NRZ data signals, the dc component must be propagated through the signal line. It should be appreciated that transmission of a data signal through an alternating-current (ac) coupled signal line is desirable and that providing for direct-current (dc) coupling of a data signal is an expensive undertaking.
Manchester-encoding of a data signal prior to transmission overcomes the above mentioned problems of transmitting a separate clock signal and of providing dc coupling for the data signal.
A Manchester-encoded data signal solves these problems by being a so-called self-clocking signal and by having no dc component. In simplest form, a Manchester-encoded signal can be thought of as being generated by combining the data signal with the clock signal. The combination is then transmitted through an ac-coupled transmission medium.
To generate a Manchester-encoded signal an NRZ data signal and a square wave clock signal are combined in an EXCLUSIVE-OR gate to produce a Manchester-encoded signal having a 50-50 duty cycle. For one-half of a bit period, the Manchester-encoded signal is at a ONE binary level, and for the other half of the bit period, the Manchester-encoded signal is at its complementary binary level. Consequently, no dc component is produced and Manchester-encoded signals, having no dc component, are well suited for transmission through ac coupled systems.
From the above, it should be recognized, that for each bit period of a Manchester-encoded signal, a transition from a binary ZERO to a binary ONE, or vice-versa, must occur. This doubles the transmission bandwidth required for a Manchester-encoded transmission system but provides significant advantages, as described above. For a square-wave clock, the transition occurs at the middle of the bit cell, or bit period. Depending upon the phase of the clock signal in a bit period, the transition from a ZERO to a ONE level represents either a ONE data bit or ZERO data bit. Similarly, the transition from a ONE to a ZERO level represents the opposite type of data bit. Because the Manchester-encoded data signal always has a transition occurring at the middle of a bit period, a clock signal can, in principle, be recovered at a receiver.
However, distortion and noise on a received Manchester-encoded signal make detection of data transitions and subsequent reconstruction of a received clock signal difficult because a receiver cannot distinguish between a valid Manchester-encoded signal and noise. This situation is particularly troublesome when a signal line has been in an idle state with no signals on the line and when a valid Manchester-encoded signal is subsequently sent. A receiver which cannot adequately distinguish noise signals from the start of the sequence of valid Manchester-encoded signals is unable to begin to properly recover the clock signal and ultimately to provide a synchronized clock signal for subsequent recovery of Manchester-encoded data signals. Thus, it should be appreciated that recovery and synchronization of clock signal for a Manchester-encoded data receiver are critical functions.
The problems of synchronization of a receiver clock and recognition of valid Manchester-encoded signal activity on a signalling line, have been addressed with a number of system solutions. For example, systems which operate in conformity with MIL-STD 1553 use a so-called synchronizing impulse data string which precedes valid data. The synchronizing impulse, of course, can be sent on a separate signal line, but obviously, that is an undesirable solution.
Another example of an attempt to provide a correct indication of line activity in conjunction with a rather sophisticated and somewhat expensive dc-coupled distributed data is a network transmission system called ETHERNET, which uses a unipolar signal format. It provides a noise filter function at its input such that input signals which are less than 30 nanoseconds duration or which are less than minus 150 millivolts in amplitude are rejected. The first pulse which is greater than 50 nanoseconds duration and which is more negative than -250 millivolts activates the receiver. The receiver is deactivated if a second such negative pulse does not occur within 160 nanoseconds of the first pulse.